A molding apparatus typically comprises top and bottom mold chases which are made of hard steel with the top and bottom mold chases mounted on top and bottom platens respectively. Molding dies are mounted on the mold chases and each molding die may include mold cavities to mold encapsulation compound onto an electronic device. The top and bottom platens are supported substantially at the corners of the mold chases by four columns (one column at each corner) whereat mechanical pressure is applied to create a clamping force throughout the molding die.
Molding is commonly conducted at a high transfer pressure of more than 6 bar followed by an even higher packing pressure to minimise voids in the molded electronic device. A higher clamping pressure along the edges of mold cavities on a carrier such as a lead frame ensures that there is no leakage of the molding compound and gives rise to more effective packing of the compound within the mold cavities. However, for molding dies that are designed to mold a matrix of electronic devices, a lower clamping pressure is experienced by the lead frame at a clamping zone near the centers of the molding dies such that mold flash or bleeding may occur for these electronic devices, whereas electronic devices along the edges of the lead frame which encounter higher clamping pressure will not encounter mold flash or bleeding. It would therefore be advantageous to enhance the uniformity in clamping pressure throughout the lead frame so that mold flash or bleeding may be avoided for the centrally-located mold cavities.
A side view of a conventional molding apparatus 100 is shown in FIG. 1. Top and bottom mold chases 12, 14 are located between top and bottom heater plates 16, 18. The bottom mold chase 14 is further supported on a packing plate 20. Above the top heater plate 16 is a top insulating plate 22. A top platen 24 is drivable onto the top heater plate 16 and the top and bottom mold chases 12, 14. The bottom mold chase 14 is supported by a plurality of supporting steel pillars 26 in the form of steel rods or springs. The supporting steel pillars 26 improve the uniformity of clamping force on the molding dies such that the lead frame is clamped more uniformly along its length to reduce leakage of the molding compound. The molding apparatus 100 further comprises a bottom insulating plate 28, a base plate 30 and a bottom platen 32.
Uneven pressure distribution may be experienced by the electronic devices using the conventional molding apparatus 100 even with the presence of the supporting steel pillars 26. Due to manufacturing constraints, parts of the molding apparatus 100 may have tolerances resulting in non-uniform gaps between the supporting steel pillars 26. The supporting steel pillars 26 on their own may not provide uniform clamping pressure on the electronic devices if there are gaps as their lengths are fixed. Therefore, the problem of uneven clamping pressure remains. Bleeding or mold flash may thus still occur for a molding apparatus incorporating the supporting steel pillars 26.
It has been found that varying the relative dimensions of the supporting steel pillars 26 may provide a more even pressure distribution on the lead frame. However, these variations may not be adequate to cater for different clamping pressures, which necessitate different degrees of compensatory deformations on the surface of the bottom mold chase 20.
To overcome the aforesaid problems, piezoelectric materials have been incorporated in molding apparatus to facilitate adjusting the height distribution along the molding surface so as to allow adjustment of the overall pressure distribution. A prior art example of an apparatus which uses piezoelectric materials to regulate the clamping pressure is described in Japanese Publication No. 09-076319 entitled “Resin Mold Device”. This publication discloses pressure adjusting means which comprises an actuator made of piezoelectric materials and which is located below supporting rods. The piezoelectric actuator acts in the mold closing direction and controls the displacement of the mold through individual piezoelectric rods. By varying the lengths of the piezoelectric rods and thereby adjusting the pressure at chosen locations on the molding die, a more uniform clamping pressure can be achieved.
Piezoelectric materials in the piezoelectric actuator should have a relatively high Curie temperature and be able to withstand a relatively high working temperature of about 400° C. that is required for molding. By applying an appropriate bias electrical voltage, the piezoelectric materials extend or contract to varying degrees on the supporting steel rods to achieve the relative dimensions which are required for a more even pressure distribution. The degree of extension of the piezoelectric material provides feedback as to the appropriate force to be applied thereto as the stiffness of the piezoelectric material is known beforehand. However, in the said prior art, the load of the molding dies, the supporting platens as well as other structures above the molding dies act directly on the piezoelectric materials. This is undesirable since piezoelectric materials are made of relatively fragile materials such as ceramic and may be easily damaged. It is therefore desirable to devise a way of changing the clamping force distribution along the mold surface without exerting a direct load on the piezoelectric materials.